In general, for a wire rope used for an elevator, a crane, or the like, it is periodically checked whether or not a break exists in an element wire. When a large number of element wires are broken, the wire rope is replaced. Such inspection is performed by visual observation as a rule, but a wire rope flaw detector may be used in order to improve work efficiency.
The related-art wire rope flaw detector includes a magnetizer and a coil portion serving as a magnetic sensor. The magnetizer includes a first permanent magnet and a second permanent magnet, which are arranged with an interval so as to be spaced apart from each other, and a back yoke configured to join the first permanent magnet and the second permanent magnet to each other. The coil portion is arranged between the first permanent magnet and the second permanent magnet.
At a time of inspection of the wire rope, the magnetizer is brought into contact with the wire rope so that the first permanent magnet and the second permanent magnet are aligned with each other in a longitudinal direction of the wire rope, and the wire rope is moved relatively with respect to the magnetizer. At this time, a segment of the wire rope between the first permanent magnet and the second permanent magnet is magnetized.
In a case where damage has been caused to the wire rope, a leakage flux occurs in a periphery of a damaged part when the damaged part enters the magnetized segment. The leakage flux is detected by the coil portion. Therefore, it is possible to determine whether or not a break exists in the element wire by measuring output from the coil portion. That is, when the leakage flux from the damaged part is interlinked with the coil portion, an induced voltage is generated at both ends of the coil portion, which enables the detection of the break in the element wire.
The coil portion includes a first search coil and a second search coil, which are arranged with an interval so as to be spaced apart from each other in a moving direction of the wire rope. Therefore, differential output between the first search coil and the second search coil is obtained, to thereby cancel a noise voltage superimposed on the first search coil and the second search coil in the same phase. Meanwhile, the induced voltage due to the break remains without being canceled because of a difference in time at which the induced voltage is generated in each of the search coils, which improves an S/N ratio (see, for example, PTL 1).